1. Field of the Invention
The present invention relates to a method of guiding and centering an electron beam in a vacuum coating chamber by means of a magnet field which extends orthogonal to the axis of the electron beam, whereby the electron beam is guided in a vacuum chamber from a cathode to a source of raw material and a burning spot is produced thereat and where by means of the electron beam the surface of the source of raw material is heated and melted and the source of raw material is fed along corresponding to the melting off of the material.
The invention relates further to an apparatus for practicing the method.
2 Description of the Prior Art
Many different methods of allowing an evaporating of coating materials and of coating the surfaces of substrates by such materials in a vacuum chamber have become known. Also known is to introduce additional reactive gases such as Nitrogen or Oxygen into the vacuum chamber by means of which surface coatings of corresponding compounds are produced. The German Patent Specification Nr. 28 23 870 discloses such a method and such an apparatus. Thereby, a substrate support which supports the articles to be coated is arranged in a vacuum chamber concentrically around the main axis. These articles may be tools such as milling cutter or drills or other parts which are to be equipped with a surface coating which shall have specific mechanical, chemical or optical properties. An electron beam stemming from a cathode arrangement and impinging on the raw materials source is guided in the center of the apparatus along the longitudinal axis of the vacuum chamber. By means of the high energy of the electron beam the surface of the raw material source is melted and deposited onto the substrates in accordance with the described method. The electron beam is focussed by means of a magnet field and aligned onto the center of the vacuum chamber. The support of the substrates and the articles to be coated located at the support of the substrates distort this magnet field and the effect of these disturbances is that the electron beam is deflected and, therefore, the raw material source is no more acted upon in a manner as really desired. Additional difficulties arise when the support for the substrates or the articles to be coated rotate during the coating process or are otherwise moved. In such a case the influences onto the magnet field change continuously and the electron beam is deflected irregularly and is disturbed. The consequence of this behaviour is that the raw material source is irregularly acted upon and melted irregularly. In extreme cases it is even possible that the focal or burning spot produced by the electron beam impinges outside of the raw material source onto supporting structures there around which may lead in case of an unsufficient cooling to a damaging of these structures of the plant. At the known apparatus a raw material source is used which is located in a crucible which can contain only a certain limited amount of a coating material.
It is also known to place in apparatuses or plants, resp. having a high rate of evaporation a bar or rod as raw material source which can be continuously fed along through the bottom floor of the crucible. This raw material rod forms the anode and if the magnet field is not influenced, the burning spot formed by the electron beam impinges onto the center of the raw material rod and causes a uniform melting of the surface. If the electron beam is deflected away from the center, the rod will meltoff one-sided and a one-sided pasty or mellow, resp. collar is formed. This one-side collar is also cooled to a lesser extent because it is at a further distance from the cooled support. The pasty part of the collar reacts with the reactive gas present in the chamber and the compound produced thereby has mostly a higher melting point than the pure material. This leads to disturbances of the coating process because of such state the rate of evaporation is changed and the melting operation is disturbed. The consequence could be that the coating process must be interrupted and that, therefore, the articles to be coated are of a lesser quality or even must be removed as waste.
The object following from the above described situation, namely to allow the centering of the electron beam during the coating process in order to produce a uniform melting process at the surface of the raw material source forms the basis of the European Published Application EP-OS-0 381 912 in which a method of centering of the electron beam by means of a magnet field rotating laterally to the axis of the electron beam is proposed. This rotating magnet field can thereby be additionally superimposed by a steady magnet field for an additional influencing and positioning of the electron beam such to erode the entire surface of the raw material source uniformly. The controlling or influencing, resp. of the magnet field for the centering or guiding, resp. of the electron beam by means of a measuring of the rate of consumption of the reactive gas in the vacuuum chamber such as proposed in the European Specification EP-OS-0 381 912 has not proven itself and has revealed itself to be unreliable and led still to a non-uniform eroding of the raw material.